1. Field of the Invention
The present invention relates to a microwave power amplifier, in particular, to the microwave power amplifier using negative feedback circuits, RC parallel circuits, and shunt resistors.
2. Description of the Prior Art
High-valued gains in a low frequency band are obtained due to a device's characteristic when designing a power amplifier in the millimeter-wave band. And, an oscillation may always occur in the low frequency band due to a limitation of modeling.
In the prior art, a feedback circuit has been employed in the power amplifier for stabilizing the amplifying device. For example, see the following publication that is incorporated herein by reference: IEEE Trans. On MTT, Vol 49 to Joao Caldinhas Vaz et al entitled “Millimeter-Wave Monolithic Power Amplifier for Mobile Broad-Band Systems” issued in June 2001, which discloses a MMIC (microwave monolithic integrated circuit) power amplifier for applying to a 60 GHz MBS (Mobile Broad-Band System). In this publication the 3-stage MMIC power amplifier was designed in two types such as a single-ended type and a balanced type by using 0.15 um PMHFET (Peudomorphic heterojuntion FET), and applies a RC feedback network to first and second stage transistors in negative feedback manner for achieving unconditional stability of a millimeter-wave transistor.
In addition, Korea patent number 2000-81018 that is incorporated herein by reference and issued on Dec. 23, 2000, discloses a radio frequency power amplifier using a feedback circuit and a method for designing the amplifier, wherein a feedback circuit is inserted in each stage of the power amplifier so that an unstable amplifying element is absolutely stabilized. It is designed by finding a peak output power point due to an output power change according to an input power of a whole power amplifier circuit, after stabilizing an unstable amplifier device using the feedback circuit in a radio frequency band.
Hereinafter, the feedback microwave power amplifier in accordance with the prior art will be explained with reference to FIGS. 1 and 3.
FIG. 1 shows a circuit for explaining a feedback microwave power amplifier in accordance with the prior art, which consists of gate bias circuit 101, 103, 105, drain bias circuit 102, 104, 106, negative feedback circuit 107, 109, HEMT (High Electron Mobility Transistor) devices 113, 114, 115, 116, inter-stag e matching circuits 108, 110, a power divider 111, and a power combiner 112.
Referring to FIG. 1, a 3-stage power amplifier utilizes feedback circuits 107, 109 in the first sage and the second stage to meet stability conditions, and interstage matching circuits 108, 110 are connected between stages to obtain interstage matching. Gate bias circuits 101, 103, 105 and drain bias circuits 102, 104, 106 are separately applied to each stage, and at the last stage, HEMT devices 115, 116 are connected in parallel with each other by applying the power divider 111 and the power combiner 112 to enhance power characteristics.
FIG. 3 shows a graph representing input/output characteristics and gain of the feedback microwave power amplifier shown in FIG. 1.
Referring to FIG. 3, when the power amplifier was designed by using only a feedback circuit, unconditional stability conditions were met in terms of stability. However, when only a negative feedback circuit is applied to the power amplifier, potential oscillation may occur at any time due to a mis-match in a low frequency band, as is shown in gain characteristics 301 and input return loss characteristics 302 in the low frequency band. In other words, the power amplifier employing the feedback circuit in accordance with the above described prior art still has a potential oscillation problem in the low frequency band.